texts: Does terrestrial planets conduct heat from near to far side?

texts:
Suppose a terrestrial planet with no atmosphere is tidally locked and very near the star like a hot Jupiter.

Is the far side still very cold or would crust would conduct heat from the near side to far side?


texts: What was the largest telescope ever fitted with an eyepiece

texts:
Serious modern telescopes use CCDs to capture the results. This makes it convenient to process or display the results halfway around the world. However, there is something visceral about looking through an eyepiece and seeing the results live.

My telescopic experience has been with smaller (<=8") devices in not-great-seeing conditions, and I've generally found the results dim and fuzzy. I've imagined what it would be like to look through an eyepiece on a serious telescope, showing a nebula bright and clear. Imagine stereo eyepieces on the Large Binocular Telescope!

Hence my question: what is the largest aperture telescope that was ever fitted with an eyepiece so that someone could look through it in real-time? This should include situations where a normally-digital telescope was temporarily fitted with an eyepiece.


texts:
Take me to the moon!

I know this is literally rocket science but, I would like any sources or formulas that would help me plot a trip to the Moon. I'm not sending anything, nor do I ever plan to, I just want to do the next best thing by plotting the trip. So if anyone could give me any info as to how I can plot the trip, that'd be great! :)

P.S. I'm perfectly aware that the moon isn't always the same distance away, that it's like hoping onto a taxi in New York City, and how it's got a lot of work (I wouldn't mind doing it really), I'd just like to plot the trip. And please don't hate on me for asking, let's just be civil adults here. :)


texts:
The Earth is a very smooth sphere, and the Sun even more so, with only minor fluctuations. I am wondering: are larger stars even rounder? Intuitively, that seems self evident, but I am not so sure. For instance, the hydrostatic equilibrium causes larger stars to be much less dense than red dwarfs. So what is the most important factor for how round a star is, a higher mass, or less activity? The most prominent cause of irregularities is of course the rotation rate of the star, which is pretty much independent of size. Ignoring that, do larger stars have smaller a deviations from the ellipsoid relative to their size?

Edit

As it seems like the "other than rotation rate" criterion is not really meaningfull, I now terminate it.


texts: Why does Pluto's Orbit underlap Neptune's Orbit

texts: Why does Pluto's Orbit underlap inside of Neptunes Orbit. Clearly Neptunes Orbit is not overlapping on Pluto's. Yet, they fail to maintain the symmetric banding appearance all the other Planets maintain, similar to the quarter mile track you often see around football fields.


texts: Have any planets been discovered in Orion constellation?

texts: Orion constellation is a very famous constellation. It can be seen throughout the earth. Orion looks like a warrior with a weapon and skirt. I see this constellation daily and wonder, if there is a planet there.


texts: What is known about the angular momentum distribution within galaxy clusters?

texts: I recently saw a 3D-plot of proper motions of galaxies within the Laniakea super-cluster (https://sketchfab.com/models/754cb85550fb42588175f8a215718521). Is a similar 3D map of the vector field of angular momentum possible? Would it be rather random, or would close galaxies within a cluster have strongly correlated angular momenta? Can we even estimate the axis of rotation from visual / spectroscopic data?


texts: Can orbital migration cause a planet to crash into the sun?

texts: If the planets move out of their current orbits into other orbits over long periods of time could this cause a planet to crash into the sun? Maybe the term crash is a little dramatic, could the orbit shrink to the point that the planet intersects with or is vectored into the sun?


texts: Has the conjunction between Venus, Jupiter, and Regulus only occurred twice in 2,000 years?

texts:
I recently heard the claim that mid-July, the "Star of Bethlehem" formed for the first time in 2,000 years, where the Star of Bethlehem is a three-way conjunction between Venus, Jupiter, and Regulus. The Wikipedia page I just linked mentions one such conjunction occurring in 2 BC, and other sources (like EarthSky) mention the conjunction that happened last month, in 2015 AD. Has this three-way conjunction never happened in the intervening 2016 years?

(For the purpose of this question, I'll say that it counts as a conjunction when each pair of bodies is separated by no more than 10 degrees, as suggested by this CosmoQuest forum thread on the subject.)


texts: Is it possible to see an image with radio telescope of something not so far away in detail?

texts:
Can an image of a person using a radio telescope see a person from far away? Could a radio telescope be made to see someone walking on Mars?

https://space.stackexchange.com/questions/30477/could-we-see-someone-walking-on-mars-from-earth


texts: Can the moon appear to be in any position in the sky?

texts:
It seems to me that the Moon has appeared in many locations - apparent locations - in the 'dome of the sky'. I know that the Sun follows an arc from east to west and that the arc is smaller in winter than in summer. Therefore the Sun cannot appear in every location in the 'dome of the sky'.
My question is: can the moon appear in any (every?) location in the 'dome of the sky'?


texts: Underground sea in Titan(Saturn's moon), water or hydrocarbons?

texts:
Is the underground sea on Titan formed of water or hydrocarbons. Is anyone certain?

I have read a lot of various documents from NASA and Cassini missions and Huygens missions, and I have become confused.


texts: Why is there a density dropoff in the stellar winds of magnetic O-type stars?

texts:
I recently went to a talk on X-ray emissions from stellar winds of O-type stars, and a paper was explored that in turn referenced ud-Doula et al. (2014).

Essentially, O-type stars may show strong x-ray emission due to radial shocks in their stellar winds. In general, the winds are isotropic, and so emission is assumed to be spherically symmetric neglecting rotational effects. However, certain O-type stars (e.g. NGC 1624-2) have strong magnetic fields; this means that outflows are channeled into rather narrow beams within a few stellar radii, which eventually spread out.

X-ray emission is characterized by a parameter $X_{T_x}$
$$X_{T_x}\equiv\rho^2\exp(-T_x/T)$$
for density $\rho$ and temperature $T$, with a threshold temperature of $T_x$. Simulations place most of the emissions in a rather narrow spot near the star, because while there are indeed high temperatures experienced all along a beam extending outward from the star, the density quickly falls off, as shown in Figure 4:

From left to right: Logarithmic density, logarithmic temperature, and x-ray emission.

Why does the density drop off so quickly? I would expect that the channeling of more of the wind into a small beam would lead to a rather steady radial density profile along it, as is the case with the temperature graph.

This is unrelated to the question, but I'll add that the temperature isn't really constant. 1D and 2D simulations of spherically symmetric show varying behavior due to repeated shocks inside the wind, and I would expect the same to be true here.


texts: What would "the next GAIA"-like instrument be like? Could it simply be a 3 to 5x scaled-up version of the same beautiful system?

texts:
This excellent, thorough and well-sourced answer to Has a gravitational microlensing event ever been predicted? If so, has it been observed? mentions several works where hundreds to thousands of events are predicted.
Roughly speaking these are within the next 50 years and require space telescopes due to the sub mas and sub mmag sizes of the astrometric and photometric microlensing effects, respectively.
The answer begins:

In the pre-Gaia era, this was effectively impossible to do, since positions and proper motions weren't precise enough for a large enough sample of stars. With the release of Gaia data, though, it has become an active area of research.

Next generation GAIA?
A simple scaling-up by a factor $f$ would have a possible smaller resolution by $f^{-1}$ so "look" a factor of $f$ farther in parallax measurements and have an $f^2$ larger light collection so would "see" a factor of $f$ farther given the same limiting magnitude (which may or may not be a valid assumption).
This is certainly a challenge for the CCD array, which may have to be a factor $f^2$ larger physically but (presumably, naively) have a factor of $f^4$ more pixels in order to fully exploit the improved diffraction limit, and that may have serious noise side-effects.
Question: Could a next generation GAIA simply be scaled up by a factor of 3x or 5x of the same beautiful system of two rectangular primary mirrors pointing 106.5 apart, imaging on the same focal plane at the same time on a big silicon carbide "optical bench" slowly and steadily rotating in synch with the CCD readout speed? If not, what would "the next GAIA"-like instrument be like?

Further reading:

Why does the Gaia space telescope have two main mirrors
Why does Gaia use only calcium NIR lines for stellar radial velocity measurements?
this answer to Why are telescope mirrors made of glass?
this answer to How are space telescopes stabilised to a perfect standstill?

above: GAIA's Silicon Carbide Optical Bench, with the two objective mirrors of it's twin telescopes pointing 106.5° apart. From Spaceflight 101, image credit: ESA/Astrium.

above x2: GAIA's CCD array, from Spaceflight 101, image credit: ESA and Astrium respectively.

above: Gaia's Imaging system, including mirrors 4, 5 and 6, prisms, diffraction gratings, and CCD array, from here, credit: EADS Astrium.


texts: What is that light of the middle of the milky way galaxy?

texts:
I'm Being confused what is that light in the middle of milky way galaxy?

Image is from Charting the Milky Way From the Inside Out

The text with the image explains a lot about the spiral arms and the location of the Earth, but the central bright area isn't explained in great depth.


texts: Does CIBER Experiment from Caltech suggest that there can be lots of stars which are not in any galaxy?

texts:
My question is about the implications of the observations recently made by the Cosmic Infrared Background Experiment, or CIBER, from Caltech. I've read at Caltech web site:

  "The total light produced by these stray stars is about equal to the background light we get from counting up individual galaxies."

Does that suggest that about a half of the stars in the observable universe could not belong to any galaxy?


texts: Frequency of gravitational wave detection

texts:
You may have heard in the news that the LIGO experiment recently detected a gravitational wave.

Though I'm not an astronomer, the paper is a good read and mostly accessible. The detection of the gravitational wave is one thing, but the black hole merger is quite new to me. From the data I could collect in the paper and this site, the source is estimated at 1.3 billion years, and the chirp lasted only a few milliseconds.

My question: what is the frequency of events of this size order?
Is there any estimate of the density of such events in the universe?


texts: What was the process of calculating how much darkmatter/matter/dark energy the observable universe consists of and how did it come at this conclusion

texts:
In 2015 the Planck satellite consortium released an analysis of data collected by the Planck satellite that showed this percentage break down of the universe’s mass/energy.

Ordinary matter: 4.9%

Dark Matter: 26.8%,

Dark Energy: 68.3%

How did they calculate the "stuff" of the observable universe and how did they come to this conclusion?

I assume  this took some tough calculations and some work with a supercomputer but i wonder what the entire process was.


texts: Why are distant objects observed in the near infrared?

texts:
I was reading an article that explains why JWST is a successor to Hubble and not a replacement for Hubble. They explained that Hubble's science pushed astronomers to look at longer wavelength. And then they said:

  In particular, more distant objects are more highly redshifted, and their light is pushed from the UV and optical into the near-infrared.

So basically to observe the first galaxies, astronomers have to observe in infrared. My question is why distant objects require observations in the infrared?

Is it because they are at a very large distance from us, so the light has lost a lot of energy on its way so it's detectable in the infrared?


texts:
When we look at the sky, and see the Milky Way, we mainly see a band, with no real center.

On a more accurate picture we can guess the center of the galaxy (see this picture), but we still don't see the center as some kind of big bright "star", like on this picture.

I know there is a lot of objects (stars, planets and dust) between us and the center of the galaxy. But stars are bright and they shouldn't "hide" the galaxy center's light since they shine too.

Why don't we see the galaxy center as a "night sun"?

How much dust is there so it's blocking the galaxy center's light?

Is it because of all Oort clouds of all solar systems that we don't see the galaxy center clearly?


texts: High quality Space-related blog/website?

texts: I don't know if this is the right place for asking this question, my apologies in advance. I would like to be updated on the latests news on space and I am looking for a website devoted to space news, or a blog. I was only able to find minor blogs which were not updated often. Could you suggest me a high quality blog/space related website that you know? Thanks!


texts: How to calculate Equivalent Width of a spectrum with continuum at 0

texts:
I am trying to calculate the Equivalent Width of emission lines of a spectrum whose continuum is at 0, making the calculation of the EW impossible.

I have the spectrum of a transient in a galaxy, so I am subtracting the galactic spectrum to study the emission lines, but, by doing so, the continuum level of the resulting spectrum is at 0. I am fitting the lines with multiple gaussians and calculating the EW as flux/continuum, but being the continuum around 0 the EW values become gigantic.


texts: Conversion of magnitudes to Jansky and MAGPHYS?

texts:
I'm a bit puzzled and not an observer, so please bear with me if I'm being stupid here.

The code MAGPHYS specifies Jansky (Jy) as the input unit for flux through a filter (see Section 3.2.3 in the documentation). Meaning MAGPHYS wants me to use Jy as unit, when specify the total light of a whole galaxy in e.g. the SDSS-g band. But Jy is a unit of spectral density, i.e. W/m²/Hz. I would expect a flux as input unit when talking about the amont of light coming through a filter, i.e. in W/m² or in $L_\odot$ (solar luminosity), so integrated over all frequencies that go through the filter and weighted by the filter response function.

If I have the absolute AB magnitude of a galaxy in the SDSS-g band, how do I convert it to Jy so that MAGPHYS will be happy?

I'm aware of:

$m_{\text{AB}\nu} = -2.5 \log_{10} \left(\frac{f_{\nu}}{3631\text{Jy}}\right) \qquad \text{or} \qquad \left(\frac{f_{\nu}}{\text{Jy}}\right) = 10^{-0.4\  (m_{\text{AB}\nu} - 8.9)}$

But that is in one frequency, not a whole band.

Additionally, I have found the formula below in TOPCAT:

$\left(\frac{F}{\text{Jy}}\right) = 10^{\left(23-0.4 \ (m_\text{AB}+48.6)\right)}$

Where does the 23 in the exponent come from?


texts:
Are there any places featuring permanent dry ice (carbon dioxide ice) on Mars?
Is there dry ice at the poles?


texts: What would happen to the Galilean moons and Titan if Jupiter and Saturn disappeared?

texts:
The Moon reportedly doesn't "need" the Earth to revolve around the Sun. If the Earth wasn't there, the Moon would continue its current path from a heliocentric reference frame around the Sun. My question is whether that would happen to some of the moons of Jupiter and Saturn too. If Jupiter or Saturn weren't there, would Ganymede or Titan continue to freely revolve around the Sun, or what would happen to their orbits?

Thank you.


texts: Is a year really 365.24 days, or is it 365.2564 days like I remember?

texts:
The NPR News item and podcast Spring Starts Today All Over America, Which Is Weird includes the following:

But why isn't the time of the equinox the same each year?
The short answer is that the time and the date are imperfect human constructs that we use to keep track of our planet's movements.
The longer answer involves leap years.
All of this is caused simply by the fact that the spin of the Earth doesn't divide evenly into one year," says Michelle Thaller, an astrophysicist turned space communications expert at NASA.
One spin of the Earth around its axis is one day. "The problem is we're happily spinning on our axis, and the Earth is going around the Sun, but one year — one complete path around the Sun — isn't an even, exact number of days. In fact, it's 365.24 [days]."

Wikipedia gives Earth's orbital period as 365.256363004 days and I have always remembered it to be 365.2564 which is the same value as Wikipedia, just to fewer digits.
So is a year really about 365.24 days or closer to 365.2564 days?


texts: Why does the Sun show different colour at midday and at morning?

texts:
As we see every day in the morning the sun shows a red-orange colour but at midday has a blue-white like colour why?

Is it concerned with the distance from the earth or about intensity? How do you see it?


texts: Say I have a flux density spectrum of a galaxy, as well as polarization spectra for the object in U, Q, and P. I want to find the polarization of some emission line in the spectrum. First, I can fit a line to my flux spectrum which will give me the continuum (i.e. without the peak(s)). I can then subtract the continuum from the whole spectrum to get just the profile of the emission line. But I'm unclear on how to relate these results with the polarization spectrum (right now I'm just working with the Q). I'm guessing I need to subtract the continuum polarization from my polarization spectrum, which would also involve converting my flux continuum to some polarization units, but how do I do this? Do I need the U or P (total) portion as well to do this?


texts:
Hopefully this is not a silly question, but I was wondering why earth is not always in the light (and any other planet for that matter). I mean, we have billions of stars just in our galaxy that all give off light from all directions. That light has obviously reached is as we can see them shimmering in the night sky.

With that in mind, what has happened to the light from the stars? Has it weakened somehow? (didn't think that was possible) Surely if we are being hit with light from all directions, it makes sense that we should always be illuminated. So what's going on here?


texts: Has star KIC 8462852's 3 year long Kepler-fading continued after the primary Kepler mission ended?

texts:
BACKGROUND

May 11, 2013 the Kepler space telescope ended its primary mission. Data captured until then showed that the now famous "Tabby's star" or "Boyajian's star" (KIC 8462852) had been fading throughout the Kepler mission by about 3% cumulatively. (This independently of other reports of brief very deep dimmings and restorations during the mission.)

QUESTION

Today when I post this question, about as much time has passed by since the primary Kepler mission ended, as that mission lasted. So this makes me wonder if other telescopes have observed, or should've been sensitive enough to have observed, a continuation of such a fading trend for that star? Another 3%.

DISCUSSION

If not, what could the explanation be for why this 3+ year fading trend was suddenly interrupted when the primary Kepler telescope mission was terminated? And how long would it take to determine from ground based observatories whether this particular dimming trend continues or not?

Fig. 5 from the paper by Montet and Simon 2016.


texts: Does anyone know about the ring diagram technique and Helioseismic and Magnetic Imager pipeline?

texts:
I'm working on the ring diagram technique and pipeline for the Solar Dynamics Observatory's Helioseismic and Magnetic Imager or HMI, but I didn't find much about the details about how it works or how to obtain the data.

I know that all starts with measuring Dopplergrams and then doing something call Postel's projection, then the apodization, then the 3D Fourier transform to obtain the 3D power spectra where there are 2 forms of adjust to each ring separately (FITSC and FITSF, both done by the HMI's pipeline).

I need more details about how all this process is done, does anyone know? Or could indicate me some references?


texts: What is the minimum distance from city that allow to see Milky Way plane?

texts: The light pollution fades the starlight coming from the Sky and this dont let people to see the Milky Way disc. Is there any study about this?


texts: What does the "a" at the end of a spectral type mean?

texts: Looking at the chart for spectral type peculiarities (here), I noticed that the letter a was missing from the chart. This seems to be an error, as the star Vega has spectral type A0Va, with the a definitely being a peculiarity. What does the a in spectral types mean, and is this actually a spectral peculiarity?


texts: Stellar mass limits for Neutron Star and Black Holes

texts:
Don't hate on me if I am asking a very basic and straightforward thing. I have a few questions about black holes and neutron stars.

What is the mass range (in terms of solar masses) for a main sequence star to end its life as . . .

A) a neutron star?
B) a black hole?

Is it there a (practically observed or proven) possible method for a main sequence star to form a neutron star (or black hole) at the end of its life cycle without undergoing through the process of supernova?

If yes, please explain or guide me some article on this matter.
What is the mass range of a main sequence star to end up as a pair instability supernova?

If the range of pair instability supernova overlaps with that of neutron star or black hole forming supernovae, how do we determine what type of end a star would have?


texts: Is there an English translation of the Parisian Alfonsine tables

texts: I'm interested in the Alfonsine tables published in Paris in the 1400s. I know there is an English translation of what survived of earlier Alfonsine tables from Castile, but that's not what I'm interested in. If you know of software that implements the algorithms implied by the table, that would be good too.


texts: Why weren't the Hubble light-echo images of V838 Monocerotis supplemented by ground telescopes?

texts:
Drawing from this answer

  The V838 Monocerotis expansion (not a supernova) and the observation of the subsequent "spectacular" light echo was quite a notable event! From Nature 422, 405-408 (27 March 2003)

The Hubble images are quite spectacular (discussed further in that answer). I've included them again below. The color images are 83x83 arcsecconds.

A later conference paper laments the fact that no HST images could be obtained the following year (2003):

  "We received HST observing time at five epochs in 2002 through DD allocations: April, May, September, October, and December. All of the observations were made with the Advanced Camera for Surveys (ACS), which had been installed in HST during SM3b in March 2002. I need not emphasize to this audience how extraordinarily unfortunate it is that no HST observations were obtained during 2003—the loss of this opportunity is truly incalculable. However, the echoes were imaged twice in 2004 through the Hubble Heritage program, in February and October. More happily, the HST Cycle 14 allocation committee did award our team observing time for an intensive HST imaging campaign from October 2005 to January 2006, and we also have two more epochs of observations scheduled in Cycle 15 for late 2006 and early 2007".

My question is: Why could't observations from one or more of the ground telescopes that are using Adaptive Optics been used to at least supplement the data in 2003? I understand they may not provide the same quality, but not even close? Not even for the polarization mapping? Is it related to the extended area of the target? Is a field of 80 arcsec too large to correct all at once? Is the pattern too complex, or is it the lack of an isolated bright "guide star" the problem?

note: I need a reasoned and hopefully quantitative answer, not your "best guess" or speculation. Thanks!

Figure 2 of the Nature paper describes the preservation of the actual light curve (history) within the structure of the light-echo shell:

  "FIGURE 2. HST images of the light echoes
  The apparently superluminal expansion of the echoes as light from the outburst propagates outward into surrounding dust is shown dramatically. Images were taken in 2002 on 30 April (a), 20 May (b), 2 September (c) and 28 October (d). Each frame is 83" times 83"; north is up and east to the left. Imaging on 30 April was obtained only in the B filter, but B, V and I were used on the other three dates, allowing us to make full-colour renditions. The time evolution of the stellar outburst (Fig. 1) is reflected by structures visible in these colour images. In b, for example, note the series of rings and filamentary structures, especially in the upper right quadrant. Close examination shows that each set of rings has a sharp, blue outer edge, a dip in intensity nearer the star, and then a rebrightening to a redder plateau. Similar replicas of the outburst light curve are seen propagating outwards throughout all of the colour images."

From Astronom. J. 135, 2, 2008 or ArXiv

  Figure 2. Images representing the degree of linear polarization, p, for each of the four epochs of data shown in Figure 1. Image scales and orientations are the same as in Figure 1. The image stretch is linear, ranging from black representing zero linear polarization to full white representing ~50% linear polarization. These images illustrate the apparent outward motion of a ring of highly polarized light in the light echo.


texts: When it was discovered that the classical planet are not stars?

texts:
The classical planets (Mercury, Venus, the Sun, Mars, Jupiter and Saturn) were considered to be "wandering stars" - and the word "planet" means just that.

Wikipedia says "By the 17th century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers."

Since the planets were considered stars in those times, the question is when did the astronomers realize that they are something else?

The same question in put in another words: When did the scientists first hypothesize that the planets do not emit their own light, like stars, and they just reflect the star from the sun? And when was that proven for the first time?


texts: How can we calculate velocity of an object given the distance from an object in parsecs?

texts:
If you had the distance from a celestial object, in parsecs, how would you calculate the velocity of the said object?

Edit: I'm sorry I don't have more context, the specific questions is: "A galaxy that is 10 million parsecs away will be moving at what velocity?"


texts: Time evolution data of the Great Red Spot on Jupiter?

texts:
Jupiter's Great Red Spot fascinates scientists for ages, so it does me. Where can I find a table (e.g. as CSV-file) with the time evolution of longitude, latitude, horizontal and vertical extension of the Great Red Spot? I am sure somebody did already invest the time to extract such a time series from all available (historical) records and images, but it
looks like I am searching a needle in a haystack.
References
There is dozends of nice images and other sources around, for instance:

Why is the great red spot shrinking?
What is behind the Great Red Spot's longevity?
How is the height of the Great Red Spot measured?
NASA, 2018: Jupiter's Great Red Spot Getting Taller as it Shrinks, NASA Team Finds
Emmanuel Kardasis et. al.: The need for Professional-Amateur collaborations in studies of Jupiter and Saturn
projectpluto.com: Great Red Spot transit times, 2021

Longitude and latitude on Jupiter is not so trivial, see F. Bagenal & R. J. Wilson: Jupiter Coordinate Systems from 2016.


texts: Are there any galaxies which fell out of sight horizon due to cosmic expansion?

texts:
If farthest galaxies run away from us with acceleration making them exceed speed of light, we should expect them to disappear from sky among time with increasing quantity. Did we observe this? Can we indicate next galaxies to eliminate and their time of decline?

My question concerns galaxies moving with all speed ranges, not only ones greater than speed of light.


texts:
A new image of deep space was released yesterday, showing tons of galaxies. One in particular (named "LEDA 14884") looks very puzzling (like an ear), zoomed in below:

Article says it's a ring-shaped galaxy. Is there a more precise characterisation in terms of well-known galaxy typologies (irregular perhaps)? Perhaps the angle is confusing it's otherwise usual shape?


texts: Where can I find data for fast radio bursts like FRB 180916?

texts:
Where can I find Strength over Time for FRB 180916?
Or at least day+time of remarkable events.

Is there some good data for other FRB that hit the earth for the last 2 years?


texts: I am wondering since neutron star is smaller than the average main-sequence stars and it doesn't produce energy via fusion so why do we still call it a star?


texts: Mercury mapping attempts before Mariner 10?

texts:
Mercury's surface was not mapped up-close until Mariner 10 passed by in 1974. This was followed by MESSENGER in 2008. I have seen several attempts of mapping Mars that came before space exploration, such as Schiaparelli's maps from 1877.

With regards to Mercury, were there any documented attempts to map its surface with ground-based observations prior to unmanned space exploration? If so, however unlikely, did any observed features from Earth match up with Mariner or MESSENGER close-ups?


texts:
I've a question on Habitable zone.

Does Habitable zone represent the region where we humans or the living beings of earth can survive?

Or is it a region where life can exist?

If it is second I want to know what are the parameters that are considered for existing of life.


texts:
A negative comment below my SciFi SE question Why did the Armageddon (1998) plot require a “rogue comet that jarred loose a Texas-sized asteroid”? says:

I don't understand the question. They needed an asteroid to hit the earth and they needed a reason for NASA to have plausibly not seen it, hence they knocked it out of its normal orbit with something.

to which I replied:

@Valorum This is 1998! That NASA might not have "seen it" is completely plausible. One of the side effects of this film was to help get NASA and ESA to step up their game and start searching for objects that may one day intercept Earth.

While a lot of asteroids were known in 1998, certainly there has been some additional effort to look at near Earth asteroids in the last few decades using existing telescopes, not to mention efforts to pu new ones in space for this purpose.
Question: How poor was our tally of objects that could produce potential extinction-level events back in 1998? Could there have been one that NASA didn't "see" at that time?

Related questions about efforts to look for dangerous objects, most of this is post-1998:

Why has the Earth-Sun libration point L1 been chosen over L2 for NEOCam to detect new NEOs?
What is the “interesting story” on the limitations of NASA's current near Earth object survey network"?
Has the National Near Earth Object Preparedness Strategy and Action Plan seen any action?
How will the “fleet of small asteroid hunters” proposed by the B612 Foundation & York Space Systems work?
Do all dangerous asteroids first pass through keyholes?
Understanding gravitational keyhole analysis for Near-Earth Objects
What is the name of the new NEO searching telescope “heavily based on” NEOCam?
Why did the Herschel Space Telescope need helium coolant?
How will the “fleet of small asteroid hunters” proposed by the B612 Foundation & York Space Systems work?


texts: Does the gravitational attraction near the surface of dense celestial objects diverge from inverse square?

texts:
Does the gravitational attraction near the surface of dense celestial objects (neutron star, white dwarf itc) diverge (to infinity) from inverse square?

This question is inspired by the similarity between EM and gravity (inverse square force.). The paper by John Lekner here (doi:10.1098/rspa.2012.0133) shows that there is an electrostatic attraction between charged spheres no matter the polarities of the charges and that it diverges at close separation until electrical short for almost all charge ratios. I am wondering if there is a similar kind of gravitational inverse square divergence for anything other than a black hole.

Actually make this for a black hole as well, although I know that a black hole is not thought to have a normal surface.


texts: Naming of the planets of the solar system

texts:
Planets of the solar system have been named with Roman mythology gods names. I have a few questions on my mind concerning that subject for a while:

Who decided to name them like this?
When did the planets got named?

Finally, the ultimate question is: if the planets got named by the Romans, implying they had not technology to observe the solar system whatsoever, why some of the names match the color of the planets?

Mars is red - Mars is the god of war
Neptune is blue - Neptune is the god of the oceans

Or maybe I am wrong (most likely) and the planets have been named more recently.


texts: Is there any planet out there which is half gaseous and half rocky ? is this possible?

texts: I have heard many times that this planet is gaseous like jupiter this planet is like super earth like rocky planet etc. so is there any planet which is mix of gase and rock ?


texts: Is it possible that the shadow of The Moon is a single dot during solar eclipse?

texts: During a solar eclipse The Moon covers The Sun on some places on the Earth. Now the focal point is a bit above the Earth so the shadow area is about 160 miles.  In history The Moon was a bit closer to the Earth but could there have been a time that the shadow area was just a dot. (probably for just some places as the Earth is not really perfect a sphere and the orbit of the Moon isn't also a perfect circle).


texts: How to calculate observer's latitude from declination and culmination of a star?

texts:
From gcseastronomy.co.uk:
A star culminates at 50°. It has a declination of +20°. What is the latitude from where it is observed?

What is the relationship between a culmination, declination, and latitude? How would one work this out?

This is the answer:
50 - 30 = 20°
Latitude = 20°


texts: How long can afterglow last after a fireball?

texts:
I was watching the YouTube video Perseid Meteor Shower - Mojave Desert, California which looks East before sunrise, presumably in mid-August of 2016.  I've made a GIF of a few seconds near the beginning where a fireball appears at about 00:03 in the video, and a faint luminescent trail appears to drift and change shape. I've timed the two stars in Orion Bellatrix and Betelgeuse and the motion is about seven seconds of video to move about 30 minutes in R.A. (near declination 0), so based on that the three or four second visibility in the video works out to about 10-15 minutes.

Is this common? Am I interpreting what I'm seeing correctly? Is such luminosity visible by eye, or only using a camera with a good lens?


texts:
From Gerry Gilmore (2018) Gaia: 3-dimensional census of the Milky Way Galaxy

4.4 Fundamental physics
Relativistic effects are highly significant for Gaia measurement accuracy, with
tests of General Relativity being a significant driver from the very start of the project.
This established tight constraints on the mission. For example, sufficient modelling of
Newtonian aberration requires that the spacecraft orbit (Lissajous orbit around L2) is
quantified with a velocity accuracy of 1 mm/s. Finite light velocity effects lead to
position-dependent propagation delays in the field of view which must be accounted for.
Monopole light deflection (the famous 1.75arcsec solar limb effect first verified by
Eddington & Dyson in 1919) exceed the microarcsec level all-sky for the Sun, and up to
90 deg from Jupiter, significantly complicating the computational effort. Quadrupole
light bending is 240µas at the Jupiter limb, and is 1µas at 8 Jupiter radii. This allows a
special Gaia experiment – to quantify light bending by Jupiter, this test involving an
oblate rotating mass moving in a deeper (Solar) potential.

The Gaia mission
(also here):

Another possible experiment is to explore light bending of star images close to the limb of Jupiter to measure the quadrupole moment of the gravitational field of the giant planet.

Of course it's easier for an existing visible light space telescope to look near Jupiter than near the Sun, and in general stars aren't particularly strong radio point sources.

How far have individual stars been seen by radio telescopes?
Do stars have "radio photospheres"? Are they different from their optical photospheres?
https://astronomy.stackexchange.com/a/36708/7982

Question: Is GAIA the only game in town for looking at quadrupole gravitational deflection of light? Is there some other method with similar sensitivity, using either Jupiter or the Sun which is less oblate but much more massive? Radio perhaps, somehow?


texts: Would you get the same planet-discovering data for our own system at the same distance we're getting from Earth?

texts:
So there's a boatload of planets being discovered around stars in our solar neighborhood.

Kepler 22, 638 ly away, was discovered to have a planet Kepler 22b, calculated to be twice the size of Earth.

The observations that make these measurements aren't high resolution images but periodic changes in luminosity (the transit method).

My question is: would a similar situation with similar telescope properties (Kepler) detect our own Earth around our Sun from a similar distance? If not, then how close would you have to be to see Earth?

Presumably there is some correspondence among: distance, luminosity/size of a star, size of a planet. Any other factors that make Kepler-22 different from us?

Note: this question asks about Alpha Centauri. I'm asking specifically about distances like Kepler-22.


texts: Scientists claim that objects left on the Moon can lie hundreds of years in the same condition like astronaut Charles Duke family photo. Why we don't see remains of asteroids inside or around craters? Are asteroids going destroyed on impact or go under the surface even without any residue? Why do we not see asteroids that hit  Moon with a small force and remained on the surface for hundreds of years like astronaut's photo? Can craters form for other reasons? For example, from underground collapse?


texts: landing on a comet is more important than landing on an asteroid?

texts:
Philae landed on comet 67P this year. And it means a big breakthrough for the science and technology of human being.

However Hayabusa mission of Japan not only landed on an asteroid but also returned an asteroid sample to Earth in 2010.

Technologically ,which is more difficult?
And why is Philae more important?


texts:
When some object is moving, it appears smaller and more massive. What if it's moving so fast it should become a black hole? Some calculations:

Let observed radius $R_0\gamma$ ($R_0$ - radius at rest) be equal Schwarzschild radius for object with observed mass $M_0\gamma$ ($M_0$ - mass at rest):

$$R_0 / \gamma = \frac{2GM_0\gamma}{c^2}$$

$$\gamma^2=\frac{R_0c^2}{2GM_0}$$

$$\frac{v}{c}=\sqrt{1-\frac{2GM_0}{R_0c^2}}$$

For Earth it gives $v=0.999999999304c$ and for Sun $v=0.999997877903c$. Is my reasoning correct? How would we see such objects?


texts: Why won't the Sun set for days at N66.2 which is below the arctic circle?

texts:
Correct me if I am wrong, but if we count sunsets by the center of the Sun apparently crossing the horizon then the Sun is supposed to set every day at latitudes under the arctic circle. (Yes if you count in the disc then adjust by 0.27 degrees.)
I was playing around with PyEphem (a python library that claims an approximately 1 arcsecond accuracy), and found that according to it the sun stays up for a couple of days even under it ($66.2<90-23.4$). Can someone explain what is going on?
I created this simple plot to illustrate the issue. The left panel shows the whole period while the right panel shows that the problem is not that the data is under sampled. The plot goes from
2021-06-13 00:12:58.085383+00:00 to 2021-06-28 23:54:05.880661+00:00.

And one more example: Kuusamo is a city at 65°58′N 29°11′E and apparently it also has a few days when the sun doesn't set: LINK1 LINK2


texts:
At the center of the Milky Way, we have a supermassive black hole. It's name is "Sagittarius A*". I was wondering that why is it called so? I mean, why "A*" why not just "A". Is there a naming system for super massive black holes?

Thanks!


texts: Could a Trappist-like planetary system exist around a sun like ours?

texts:
So, a couple of years ago, the TRAPPIST sun was discovered to have 7 different earth-like planets orbiting it – three of them in the habitable zone. Nothing like this has ever been seen before, or found again since. And the main reason it's possible at all is because of the orbital resonance of the planets – they orbit in a pattern which causes the system to be stable long-term. And this whole system of planets take place closer to the TRAPPIST star than Mercury orbits our sun.
This is interesting for a lot of reasons, but one that appealed to me was that so many planets could fit – and be stable – in such a small space. I was under the impression that you couldn't fit that many planets so close together.
(Another interesting fact: The distance between TRAPPIST-1B and TRAPPIST-1H is less than the distance between the two edges of our sun's Goldilocks zone.)
So my question is this: could such a system be stable around a larger sun – a sun like ours? And could such a system work with the planets further out – in the habitable zone? Would it be stable or am I missing something?


texts: what will be the exact position of the Thirty Meter Telescope

texts:
TMT will be put at Mauna Kea in Hawaii. It should be near Keck and Gemini.
However it seems there is no place left at that site.
What is the exact position?


texts: How can we estimate if a certain astronomical object will be visible from a place on earth?

texts:
If we have the RA/DEC of an object that we want to observe, how can we tell if that object is visible from our place on earth (at a particular time of the year)?

Given that the part of the sky we are observing is changing throughout the year, it seems fairly complicated.


texts: What's still needed before we can observe orbits of exomoons thereby weighing exoplanets?

texts:
Comments below this answer to How do we weigh a planet? point out that we currently cannot (or at least have not) detect moons around exoplanets, much less measure the sizes and periods of their orbits around their planets.
I commented there that:

...detecting exoplanets wasn't possible, nor did direct imaging of planetary systems, nor did making parallax measurements from the Kuiper belt, nor was the detection of gravitational waves...

my point being that impossibly challenging observations keep getting done.
So I'd like to ask what the most likely extension of observational Astronomy would be that would allow for the measurement of the period of exomoons thereby weighing exoplanets.
Is it simply a pair of space telescopes in a halo orbit with laser interferometry between them and plenty of time? Could it be done from Earth? After all, exoplanetary systems are already imaged from Earth.


texts: Is the expansion of the universe proof of the big bang?

texts:
Is the expansion of the universe proof of the Big Bang theory?

Or are there other proofs?


texts: Is there a mechanism that makes small moons more rounded than comets?

texts:
67P/Churyumov–Gerasimenko has a highly irregular dumbbell shape. But the sample of comet shapes observed is very small, so I wonder if irregular shape is the norm for comets and for small moons. Many known moons are no larger than this ~4 km diameter comet. The smallest moons imaged are modestly irregular, basically just elongated, at least they don't have a waist like 67/P. The most elongated moon is perhaps ~135/60 km Prometheus.

Is there reason to believe that moons which are too small for hydrostatic equilibrium get more rounded than comets of similar mass and composition? Moons differ from comets in several ways. AFAIK: Moons in general have a very different gravitational environment, a much more stable distance to the Sun, experience more frequent impacts, another formation history if not captured. Composition and density depends on the formation distance from the Sun, although even ~300 km Hyperion has a similar density as 67/P.

Should we expect the small moons in general, and the moons of Pluto especially, to be rounded or dumbbell shaped? If 67P is a merged binary, isn't it more likely for two objects to merge if they are moons instead of comets, since neighboring moons have low relative speed? ~100-200 km Janus and Epimetheus look as if they could merge fairly calmly. Does the lack of (observed) dumbbell shaped moons tell us something about 67P, that its shape is a rare outlier for example?


texts: Why are Saturn bands much fainter than Jupiter's?

texts:
The atmospheric composition of both planets are very similar. Then why are Jupiter's bands more visible (North and South equatorial belt, etc.):

Whilst Saturn's are not:

(except for the storm of course)


texts: What is the underlying nature of the dark spot found on Uranus?

texts:
What is the nature of bright spots found on Uranus? actually quotes Space.com's Uranus Has a Dark Spot which says:

During the past decade, many bright spots have been seen on Uranus in both red and near-infrared filters. But this is the first dark spot ever seen on the planet.

Question: So I'd like to know if there is any science-based thinking on the underlying nature of the dark spot on Uranus beyond "it looks dark, like the ones on Neptune". Has this all been sorted out or is it still mostly a mystery?
Wikipedia's Climate of Uranus; Uranus Dark Spot gives a lot of descriptive information and relates the spot to similar things seen on Neptune, and describes some speculation, but this article does not seem explain much.

Just as we near the end of the hurricane season in the Atlantic Ocean, winds whirl and clouds churn 2 billion miles away in the atmosphere of Uranus, forming a dark vortex large enough to engulf two-thirds of the United States. (Image credit: NASA, ESA, L. Sromovsky and P. Fry / University of Wisconsin)


texts: How many (or percentage) satellites of Solar system planets have their orbits synchronized like the Moon (always facing earth)?

texts: Mercury and Venus have no satellites. Then earth has moon (synchronized orbit). For Mars - both its satellites (Fobos and Deimos) also synchronized. Analyzing other planets is hard - too many satellites. Maybe someone knows percentage - all of them synchronized or which ones are not synchronized (if just a few ones are not synchronized). All 4 largest Jupiter satellites are also synchronized.


texts: What percentage of Helium-3 is primordial vs produced in stars

texts:
I tried to research but what I found is pretty limited.    A very tiny but non zero percentage of primordial matter was Helium-3 or 3He.

Stars produce 3He as part of the proton-proton chain but they also consume 3He.   It has about a 400 year half life in our sun.   From Wikipedia.

  In the Sun, each helium-3 nucleus produced in these reactions exists
  for only about 400 years before it is converted into helium-4.[6] Once
  the helium-3 has been produced, there are four possible paths to
  generate 4He

My question is 2-fold.   Is the primordial amounts of 3He significant or insignificant compared to what stars produce and eject by coronal mass ejections or blowing up into nebulas, and are there specific stars, due to the internal heat and rate of reaction that product and eject more 3He into their mass ejections.

For example, would airless, rocky, magnetic field free worlds around red dwarfs be more saturated with 3He or would you find more around larger, hotter stars that undergo fusion faster.    If I wanted to go He3 mining, would I set my ship to a red dwarf system or a blue star system or a nebula?

It's not for a book or anything, I'm just personally curious, as 3He is potentially very useful stuff.

I'm aware that primordial isn't particularly easy to collect, as anything gaseous and primordial would spread out and only collect in large gravity wells, gas giants or larger.   But primordial 3He could, for example, exist in Jupiter or Saturn from formation, though the formation of their magnetic fields probably prevents them from absorbing any ejected from stars.  Hence the primordial vs star ejected ratio and the type of star that produced the most question are somewhat related, so I thought one question rather than two, but I can split into two questions if desired.


texts:
I am writing a work of fiction about a world where:

The length of a year is 335 days.
The world has two Moons.
Moon A is Larger and further away with a 78 day lunar orbit.
Moon B is smaller and closer with a 31 day orbit.

How often would both moons be full at the same time, overlapping each other like a bullseye?
How often would lunar eclipses occur for Moon A?
How often would lunar eclipses occur for Moon b?
How often would both eclipse at the same time?

This is a work of fiction, so liberties can be taken.  I just want it to be close.


texts: I have a question. I studied Bondi's model for spherical accretion. I know the assumptions made in the model are big and that they are rarely representative of what happens in astrophysics. What I wanted to ask is: is there a situation that suits this model even approximately, or it is a total idealization? Is there always an accretion disk, or may there be a situation where the accretion is spherical?


texts: Calculating effective SSN (sunspot number)

texts:
I am doing research on SSN-like parameters. As I can see, effective SSN is a parameter that is derived from SSN or other parameters. Do anyone know how to calculate effective SSN?

For example, this file has some parameters. See also this page.


texts: Is the magnetic field strength of a star predictable from mass, composition, age and angular momentum alone?

texts:
This answer to Which stellar properties can we describe as “first principles” in which we can derive the rest? contains an intriguing bit:

That states that composition and mass are the two key properties, assuming that the star is in hydrostatic equilibrium and derives its energy from nuclear reactions.
I agree with Rob that rotation (angular momentum) is a key second-order effect, and I'd add magnetic field strength as well.

which triggered an intriguing comment:

Is the magnetic field strength generally independent of the other three properties?

Question: Is the magnetic field strength of a star predictable from mass, composition, age and angular momentum? Or is it somewhat random or at least unpredictable from those alone for some reason?
I suppose it could be somehow inherited, influenced or acquired from its surroundings during formation, or arise from some random or chaotic process, but I have no idea.


texts: Why do certain moons have their rotational period equal to their orbital period?

texts: Our moon and Saturn's moon, Titan, have this feature.  Because of this we only observe one side (hemispere) of our moon. Why is this?  What is the Newtonian or astrophysics that would explain this?  It seems that it would be some kind of equilibrium that is arrived at over time, but I don't understand it.


texts: When could the Roche limit be larger than the Hill sphere?

texts:
This answer to Are Saturn's rings stable? begins with:

  Most of Saturn's rings are inside it's Roche limit, which means they will never clump together. Tidal forces prevent this from happening.

I always confuse Roche limit and Hill sphere, so I thought I'd ask a question about both.

Question: Under what conditions could a solar system body orbiting the Sun have a Roche limit larger than its Hill sphere?


texts: Why are retrograde orbits more stable than prograde ones?

texts: Regarding solar system dynamics, i.e. planets in stellar systems and moons in planetary systems, this is often mentioned in the literature, but it is difficult to find a good analysis/explanation of this phenomenon.


texts: How to convert celestial Coordinates with a radius, to max and min celestial coordinates?

texts: I am writing a piece of python code to convert right ascension and declination with a given radius to minimum and maximum right ascension and declination. I firstly can't understand why the min and max is not always just the given coordinates +- the radius, and secondly how I would write a piece of code to make this conversion. Cheers.


texts: "If earth had rings like Saturn": A few further thoughts on the consequences

texts:
I came across this video recently where artists depicted what it would look like if Earth had rings like Saturn. http://www.wimp.com/earthrings/

Please note: The video contains already some information on what I ask here, but I can't assess its reliability, therefore I'd appreciate any scientific thoughts on it :)

1) Earth has roughly a distance of 1 AU to the sun, Saturn between 9 and 10. Sun's gravity would therefore have a much stronger influence on these rings. Would it already be enough to distort the rings (e.g. even leading to the sun "stealing" parts of it) or would the rings still have a ring shape?

2) How well would the objects in such rings be visible from earth? The Saturn rings are mostly dust and ice as far as I know, but there are surely some larger objects. Depending on material and size some object would probably be visible to the naked eye, right? Is there (probably yes) a formula to calculate what materials and sizes are required to achieve this?

3) Would those rings not be constantly raining particles down on earth? Would there be a much higher risk of dangerous impacts for the earth's population?

4) Are the colours used in the video precise? Don't they depend on the angle? What influence would our atmosphere have onto them?

5) Would those rings have the same orbital inclination or would theirs differ from earth's to an extent where the difference becomes visible?

6) What shadow would these rings cause on earth? Would they be dense enough to "shade out" complete stretchs of land?

I'm aware that these are hypothetical questions. But I find it, especially as an author of sci-fi and fantasy, nonetheless very interesting.

Thank you for your interest!

Pat


texts: How does the gravity of one planet affect the velocity of other planets?

texts: If there was a massive enough planet outside of earth's orbit to affect it, how would the earth's velocity change? I know it would be small, but would the velocity increase or decrease?


texts: Why can I see the whole moon during various non-full-moon phases?

texts:
Here's an image that describes the phenomenon I'm asking about.  The very thin sliver to the left is of course the surface of the moon as lighted by the sun.  But the rest of the moon is also faintly lighted -- by what?

I'm guessing that some amount of "earthshine" -- sunlight reflected from the earth -- is the culprit here, but I've found no authoritative confirmation online. It would be great to have a link to a source that explains what's really going on.


texts: What is the process for vetting the 4,175 candidate planets Kepler has discovered?

texts: In the National Geographic article about NASA's announcement of new planets discovered by the Kepler spacecraft, it states that there are 4,175 candidate planets. What is the process for vetting these candidate planets? How do we know they exist without knowing whether they are planets? Or are they candidates because the data hints that a planet might be there but we need more detailed data or more analysis before we can confirm it?


texts: Why can we trust Hubble Time if the rate of expansion is not constant?

texts:
The age of the Universe can be estimated from taking the inverse of the Hubble constant: $t_\text{universe} = 1/H_0 =d/v.$

It seems to me this method assumes that any given galaxy has been receding at a constant apparent velocity for the lifetime of the Universe. However, by including data from larger & larger distances, it can be seen that Hubble's law doesn't hold. The data suggest that the rate of expansion was actually slower in the past.

Why is it appropriate to use only nearby galaxies to estimate the age of the Universe using the Hubble time when the rate of expansion isn't constant?


texts: Is there a theory about a collision between a shrinking universe and a new expanding universe?

texts:
I recently read a post on a discussion board about the creation of our current universe that stated, and I quote:

  [T]he shrinkage activity of the old universe and the new Big Bang expansion movement still made a tremendous collision, resulting in the diffusion and spiral of fine particles.The diffusion of fine particles was integrated into a swirl to reduce the collision and the thermal activity became mature, but the expansion of the fine particle spiral group by the Big Bang is still progressing, expanding while colliding with the shrinking old matters of the old universe There. New matters cooperate solidly with a spiral net to the whole universe as a gear that rotates at the same speed.

I am just an amateur interested in astronomy, but I have never heard of a theory regarding such an event before. Is this a known theory that has been researched?


texts: What would happen if the earth's orbit were slightly perturbed?

texts:
Suppose a rogue planet came zipping through the solar system and changed the earth's orbit a little, including the plane of the orbit.

I have the idea that, in a complex system, orbits would tend toward roughly circular and in the same plane. In simulations of star clusters I've seen, they all whip around in a crazy dance, some of them flung from the system entirely, and the remainder in roughly circular orbits. In our own solar system I am vaguely aware that the stability of the planetary orbits has to do with the relations of the orbital periods, resonances, and that the early solar system was a chaotic place, with Jupiter wandering in and then out again, until everything settled down. Just because the primordial cloud had an average angular momentum doesn't mean the planets must have all originally orbited in the same plane.

So I have the idea that in a complex system, where the different bodies can affect each other's orbits, they would tend to become circular and in a plane. Comets and asteroids by contrast, besides having long orbital periods, can't change the planetary orbits to any meaningful degree, they're flying through predetermined gravitational fields that don't adapt to them. But if the earth's orbit were perturbed, I'm supposing that the inner planets would adjust and the earth's orbit would recircularize, even if it doesn't settle down to quite its original orbit.

I have to think that astronomers have simulated solar systems and come to some conclusions about that. Is anyone familiar with that and can say something about it?


texts:
Earlier, I was looking for the current distance from Earth to 67P/Churyumov-Gerasimenko, in order to do a "back of envelope" calculation of signal delay. Not important, but I was somewhat puzzled to find quite a large discrepancy between values reported by different sites, and would love to understand why.

ESA's Where is Rosetta page currently gives the distance as around 719m km, which, if I'm using it correctly, is backed up by JPL Horizons tool, which gives me a delta of around 4.81 AU. Which is great.

But when I look at The Sky Live's 67P live tracker, it is reporting around 941m km, which their comet info page gives as around 6.3 AU. Am I missing something blindingly obvious?


texts: If a solar system were surrounded by a cloud of debris, is it possible for a planet's orbit to intersect it?

texts:
I'm doing research for a story that I'm writing and I'd like some physics advice on whether a hypothetical system of celestial bodies is possible.

Suppose the following:

A system of planets orbiting a star, similar in size to our own.
A cloud of debris (Asteroids, technological refuse, and dust) surrounds the entire system, similar to the description of a Dyson swarm.
The outermost planet orbiting on a path that intersects the cloud during a small portion of it's year, but is within the cloud for the majority of it's year.

Is this configuration possible within our understanding of physics, or would gravity cause the objects of the cloud and the outermost planet to 'snap' to an orbit that does not intersect? Would the planet 'capture' objects within the swarm in it's own orbit?

My story takes place on the inside of a Dyson swarm or bubble, where interplanetary travel has developed to a reasonable degree, but cannot escape the swarm. An area of intrigue might be the outermost planet. If it can be landed upon while it's orbit is within the swarm, people could 'ride' the planet through to the other side if they stayed there for a number of years or went into stasis there.

Any thoughts or opinions are greatly appreciated.


texts: Challenges when trying to build a telescope using two lenses

texts:
I have some lenses - a 10 cm and 7 cm convex lense and a 4 cm and 1 cm concave lenses. I am trying to make a telescope by combining one of convex and concave lenses, but I am struggeling to do so. The picture that is created using these two lenses doesn't show anything other than the thing that can be seen by eyes. Is it possible to make simple telescope using these lenses?
PS: All of numbers are lense diameters.


texts: Building a floating, ocean-going giant radio telescope?

texts:
Given the loss of the Arecibo telescope: Is anybody aware of any efforts or studies towards a floating, ocean-going radio telescope of with 500m diameter and more?
The idea is floating (pun intended) in my head since I heard about Japanese design studies about foldable aircraft carriers (actually it is just the runway which is expandable and thin).
In the age rockets self-landing on a small sea-born platform, I assume that we have the wave movements under control, at least in principle.
Related

Is there any role today that would justify building a large single dish radio telescope to replace Arecibo?
Could we carve a large radio dish in the Antarctic ice?

References

Osaka University: Department of Naval Architecture and Ocean Engineering
Wikipedia on the Drilling platform "Erik Raude" (in German). This shows that even in rough seas, massive vessel canbe kept extremely stable and static.


texts: Helium rain inside metallic hydrogen within Jupiter, conditions and experiments?

texts:
In response to today being pi day (also see http://www.piday.org/), NASA has published it's fifth annual pi in the sky day 5 activity set (see also NASA goes the distance and Celebrate Pi Day with NASA).

This year's NASA activity includes the problem set Helium Heist: A 'Pi in the Sky' Math Challenge. Below is a cropped screen shot of the illustration from this PDF version.

The part of the text that I found most striking is:

  It has been hypothesized that the helium was depleted out of the upper atmosphere and transported deeper inside the planet. The extreme pressure inside Jupiter condenses helium into droplets that form inside a liquid metallic hydrogen layer below. Because the helium is denser than the surrounding hydrogen, the helium droplets fall like rain through the liquid metallic hydrogen.

Question: Roughly what are the pressure and temperature thought to be at this depth in Jupiter? Is this regime at all accessible experimentally on Earth to confirm that these two fluids are immiscible, or is this behavior purely theoretical?

below: cropped screen shot from here (NASA). Click for full size.


texts: Have we ever observed a large meteor hit the Moon?

texts:
Since the moon has no atmosphere, its surface has many more meteorite impacts than Earth's does.

Image source

Have we ever observed a meteor strike the surface of the Moon?


texts: Math behind a uv plot in interferometry?

texts:
I've seen standard uv plots used discussions of interferometric array designs regularly, and I have a vague idea that each arc may correspond to a pair of elements within the array, and the coordinates $u, v$ in the plot may be something like the path length difference between the source and the two elements projected into some cartesian coordinates, and that the length of the arcs corresponds to the total duration of the observation, but it's all still fuzzy.

Since the elements may be on topography or be so far apart (as in the example below) that the curvature of the Earth is even larger, the formal definition of the $uv$ plane should take this into consideration.

There are several more examples of uv plots and comparisons to point spread functions in the SlidePlayer presentation Array Design and Simulations.

Question: What is the math behind the construction of $uv$ plots in radio astronomy? A source that explains it and shows how one can construct them would be extremely helpful, as will any intuitive hints.

below: From the recent ArXiv preprint Detection of Intrinsic Source Structure at ~3 Schwarzschild Radii with Millimeter-VLBI Observations of Sagittarius A* (also ApJ Open Access.

below: From the 2016 ArXiv preprint High-Angular-Resolution and High-Sensitivity Science Enabled by Beamformed ALMA


texts: I read an article mentioning that when two stars orbiting each other they'll produce gravitational waves that can carry away energy into space. These phenomenon increases the speed of the two orbiting stars and draws them closer to each other until they finally collided, is it true that gravitational waves can carry away energy into space?


texts: Why is NEOWISE used for galactic observation?

texts: I've been studying images from the WISE and NEOWISE missions. WISE used infrared cameras to survey the entire sky. When its primary mission ended in 2010, it was repurposed to look for Near-Earth Objects (described here). What changes to the spacecraft and software were needed in order to change its mission profile? Does it use the same cameras and instruments now as it did when performing a whole-sky survey? My thinking is that a whole-sky survey requires different equipment than a survey to look for small, fast-moving (compared to the stars) asteroids.


texts: What type of camera instruments are used to record optical images in space telescopes like HUBBLE & JWST.?

texts:
I am just curious about photos taken by space telescopes

What kind of type of camera systems are used in space telescopes like HUBBLE and JWST?

Are those cameras systems specially built for that purpose?
Is there a significant difference between cameras we use and of those?

What type of lenses are used?


texts:
I would like to re-align an image using a reference image (taken from GAIA). I am aware of the software Koords developed by the Karma team: https://www.atnf.csiro.au/computing/software/karma/

Unfortunately, the software crashes (with segmentation fault) everytime I try to add a source in either image. Is there a more up-to-date version of this tool? Or simply a different tool?

Update: Thank you for the responses! I will try to better explain exactly what I am trying to do :)

I have an X-ray image taken from Chandra and a GAIA-Aligned HST image. I know that the pointing for the X-ray image is off by ~.5 arcsec or so. I want  to re-align the X-ray image so that any X-ray point sources match their optical counter parts using the GAIA-Aligned HST image as my reference.  I hope this helps!


texts: Are there any known objects in a "dual" tidally locked orbit?

texts: Is there any known pair of objects tidally locked with each other?  I know the moon is tidally locked with the earth (e.g. one side always facing earth), but I don't know of any pair of objects that are both tidally locked with each other.  For example, the planet would be tidally locked with the moon, and the moon would be tidally locked with the planet.


texts: Regarding core fragmentation of a gas giant in the envelope of a red giant

texts: We know that large gas giants, such as Jupiter, have degenerate cores. Let's say there is a hot Jupiter ($5M_J$) that orbits an F-type star. When the star expands into a red giant, the hot Jupiter's gaseous envelope is vaporized. If the degenerate core (made of metallic hydrogen and some terrestrial material) fragments during the star's red giant stage, and the red giant quickly becomes a white dwarf after that, we'd get some solar system like Kepler-70. But what about those degenerate planets made of metallic hydrogen with metal impurities? Would they still be degenerate, or would they slowly/quickly lose their hydrogen to become a super-Earth mass object?


texts: Energy needed to create a black hole vs energy needed to run the Large Hadron Collider?

texts:
I was disappointed to hear Brian Cox's flippant reply to whether or not the LHC could create a black hole big enough to swallow the earth in this video:

https://www.youtube.com/watch?v=QFy87tFTZwY

I'm guessing that the power used by the LHC is nothing like that used when gravity crushes a star to a singularity, but ...

... how big is "nothing like"?


texts: What is the degree of ionization is the solar photosphere?

texts:
I am wondering how many free electrons per baryon and how many free electrons per atom there are in the solar photosphere.  This number depends on the abundances of the various atoms found in the photosphere as well as each atom's ionization potential.

The book "Introduction to Stellar Astrophysics" by Boehm-Vitense, vol. 2, 1997 reprint, on page 76 claims that one out of every $10^4$ hydrogen atoms are ionized, so hydrogen is mostly neutral in the photosphere; however, there are other heavier elements with lower ionization potentials.  Boehm-Vitense goes through and calculates the fraction of ionization of iron but calculations for only two species present in the photosphere can hardly be considered comprehensive.

Whether you calculate these numbers yourself or provide a summary of a reference (with a link to that reference if possible) that has more details about ionization in the photosphere it will be appreciated.

My guess is that, since hydrogen has such a low degree of ionization, the number of electrons per baryon and per atom will be small, since hydrogen is the majority constituent of the photosphere.


texts: Strong orange line in wood fire spectrum?

texts:
I just built a DIY spectroscope using a CD and after making some tests, I noticed a really strong emission line located in the orange-yellowish zone of the fire spectrum (specifically, burning wood).

I think that the strong line could be actually sodium, but wouldn't that mean that wood contains a lot of sodium in order to see such a strong emission line?


texts:
First of all, I have very basic knowledge of astronomy so sorry if I make some mistakes in my terminology.

I'd like to plot the Southern Cross. I can find the name of its 5 stars. Also, I have some knowledge of Python and I've tried to use PyEphem and AstroPy.

What I don't know is: what kind of information do I need to plot the cross on the circle? I mean, I'd like to plot it exactly at the right position.

The most difficult part is: the observer should be in Rio de Janeiro and the date should be 1889-11-15 08:30:00 UTC-03.

I tried to collect some info with the Astropy function FixedTarget.from_name() but, if I understood correctly, the date is too far in the past.

If anybody could give me some guidelines (maybe an online tool) to create a sky map with the cross in the right position, I'd really appreciate it.


texts:
I've been trying to figure out how to find the diameter in kpc of any galaxy given that one knows the distance to it in Mpc (d) and also the angular diameter/size (A) in arcsec. I had a formula before to find the diameter of a planet: $D=\frac{2\pi dA}{360degrees}$ but this is only if the celestial object is round. Then I asked my friend who gave me another formula $D=d*10^6 *sin (\frac{A}{3600})$

My question is, is the second formula the correct one if I want to find the diameter of any galaxy no matter what shape it is or is there another one which I do not know of?


texts:
Each new star we find is generally considered to be part of the constellation it is nearest to.

Our Sun is obviously a star, just much closer. Is our Sun part of any constellation? If so, which constellation is it a part of?


texts: What is meant by the notation $A^{\times{B}}_{\div{C}}$; where A,B,C are real numbers?

texts:
I have come across a few papers using the notation $A^{\times{B}}_{\div{C}}$; where A,B,C are real number. For example, $3000^{\times{3}}_{\div{4}}$.
An example can be Eqn (4) in Stern & Laor (2012) "Type 1 AGN at low $z$ – II. The relative strength of narrow lines and the nature of intermediate type AGN", which reads as follows:

$$\frac{L_{\rm bol}}{10^{43}\ \rm erg\ s^{-1}} = 4000^{\times 4}_{\div 4} \left( \frac{L_{\rm [O\ III]}}{10^{43}\ \rm erg\ s^{-1}}\right)^{1.39}$$

What is the generic meaning of this notation?


texts: Does this smartphone photo show Mars just below the Sun?

texts: I live in Essex, England (51.7678° N, 0.0878° E). On 25 July 2019 (hottest day ever in the UK, btw), 06:43 BST, I took this smartphone photo of the Sun. Is that white dot just below and left of the Sun the planet Mars? I've checked with my SkySafari app and it looks like it could be Mars, but I'm just surprised that my relatively cheap phone could take a picture of the planet. Although the photo is quite dark, it was a bright day. Thank you.


texts: What are some dark features on (or qualities to) the moon?

texts:
I know there are mare, oceani, and lacunae, but anything else that I'm overlooking from lists of lunar features?

Does the "dark"/far side have a particular, maybe Latin scientific name (tho it gets lit as much as the near side, of course), or the unlit area aside from 'night'?

(I'm trying to name a kitten who was born July 19th during the Apollo XI 50th anniversary. All girls, the one white one with soft black extremities is Luna, one all black but the barest white dusting about her neck is Mare, then there's Mare's essentially identical twin, who needs the name.)


texts: Does the sky darken proportionally during a partial solar eclipse?

texts:
During a solar eclipse, some of the visible area of the sun is obscured by the moon.  The exact percentage varies with time and with the observer's location.  My question is, does the sky darken by this same percentage?

In other words, suppose the sun is 80% covered.  Does sky light diminish by 80%?  Are objects illuminated by 20% of their normal amount?


texts: When and how was the length of a Mars year determined?

texts:
I guess it is known for a long time that Mars has an orbital period of around 687 days.

But when and how was this determined?


texts:
Wikipedia's Cyclopropenylidene; Titan (Moon of Saturn); Formationsays:

The major formation reaction of c-C3H2 is the dissociative recombination of c-C3H3+.
C3H3+ + e− → C3H2 + H
c-C3H3+ is a product of a long chain of carbon chemistry that occurs in the ISM.

From that I conclude that Cyclopropenylidene in the ISM (interstellar medium) is also a product of a long chain of carbon chemistry.
**Question What "long chain of carbon chemistry" in the interstellar medium produces cyclopropenylidene? (C3H2)

note: While Detection of Cyclopropenylidene on Titan with ALMA is interesting news, my question is about the ISM, not (necessarily at least) Titan's upper atmosphere.


texts:
In films such as Interstellar, 2001: A Space Odyssey, and Contact (and also Star Trek Deep Space Nine) wormholes are depicted as tunnels along the edges of which fantastical colours and shapes lie. For example, this is a picture from 2001:

What would passing through a wormhole be really like though?


texts: What are those long, smooth, longitudinally-opposing areas from pole to pole on Mercury?

texts:
Look at this model and spin it - Mercury 3d model

Are those artifacts of stitching together images, or does Mercury really have these opposing, huge pole to pole lines of smoothness?


texts: I watch the discovery science channel a lot & enjoy the material about space, stars, planets, etc.  In one episode the narrator states our Sun will one day become a white dwarf star.  The size of our Sun will double and ultimately the Earth will be inside this enlarged Sun. This the end of all life on Earth.  Then I read a recent Snapple bottle cap that stated our Sun is really white.  I read it again and chuckled: is it already a White dwarf star at this point?  I take it that the original color of the star happens to be white.  How will we know when the Sun will become a White dwarf and increase in size the engulf the Earth? Basically what is the time frame in this context.


texts: What would happen if a rogue planet hit one of the planets in our Solar System?

texts:
As there are millions of rogue planets in the Milky Way, what would happen if one hit a planet in the Solar System.

For example, if the rogue planet hit Pluto, would Pluto fall in to the Sun due to to its gravity? What kind of catastrophic damage could it do in the Solar System?


texts: Relation between inner and outer eccentricities in a hierarchical triple star system

texts: According to Table 3 of Sterzik & Tokovinin (2002), the inner eccentricity of a triple star system (i.e. the eccentricity of the inner binary) is, most of the times (70%), larger than the outer star eccentricity. This seems counterintuitive, since I've always thought the outer eccentricity was usually larger than the inner one. Any reason for this? Or is this data wrong?


texts: Has SETI data been used for astrophysics? How, or why not?

texts: SETI collects alot of data from space, mostly radio. I wonder if this data has any astrophysics value other than SETI?


texts: How to generate times of sunset along a line that crosses a continent?

texts:
Would like to avoid using single location iteration of a website that gives time of sunset each date for locations.

I want to be able to produce a chart or info-graphic for publishing on a website as a guide for participants stretching from nyc,through Lebanon KS,to San Francisco.

A great circle path with formula or algorithm to produce the table or graph???

interval of time would be June to November only


texts: I'm having trouble achieving sharp telescope focus

texts:
I've been having trouble getting really sharp focus on my telescope. Using Saturn as a reference, with a 20mm eye piece it is very sharp. But when I go to 10mm or 6mm, it gets bigger and blurrier. I re-adjust focus each time I change eye pieces, but the best I can get at the stronger magnifications is still very blurry compared to 20mm. The 20mm will have nice sharp edges on the planet and rings, whereas 10mm and 6mm have fuzzy edges on the rings, and the planet itself.

I'm using the Celestron NexStar Evolution 8 Schmidt-Cassegrain Telescope. I'm in a suburban area so I'm sure there is light pollution, could that be a factor?

Last night I was observing for about an hour, which I believe would be enough time for the temperature of the telescope to match the temperature outside.

I also tried combining the 20mm with a Celestron 2x Barlow lens, just with an idea that maybe it would make a difference, but it was pretty much the same.

Is there something I'm doing wrong?


texts: What are the correct positions or alignment of planets in relation to each other and the Sun, is it vertical center, baseline or completely random

texts:
From a lot a illustrations and videos, the planets seem to align in vertical center to each other like this:

Is this really true? I like to imagine that the planets are actually in weird alignment to each other, for example, maybe Venus is 50,000Km below earths bottom etc


texts: Can all astronomical signals be considered Gaussian noise?

texts: While studying the techniques astronomers use to mitigate radio frequency interference (RFI) I found it's often assumed that astronomical signals are (band-limited) Gaussian noise. Is this a reasonable assumption for every type of astronomical observation? Even when searching EM emission from specific elements (e.g. hydrogen line)?


texts: How do we know that black holes are spinning?

texts:
How is it possible to know if a black hole is spinning or not?

If a planet is spinning, you can see it clearly but you can't really see a black hole.

Next thing would be that matter interacts with adjacent matter and we could see in which direction the matter surrounding the BH spins (like if you spin a ball on water, the water around would spin too in the same direction) but matter can't interact from inside the event horizon to the outside, so matter right at the event horizon would just be interacting with gravity (like the BH has no friction).

Now gravity. I would think that you could measure the differences in gravity if a large object is not perfectly uniform but I think a BH has the same gravitation pull on all sides.

What am I missing here? How can one even detect or determine by observation that a black hole is spinning, or better yet, measure how fast?


texts:
I'm having a hard time understanding this image in the CNN News item This glowing ring of light is from a distant galaxy.
The caption (from this version of the article) says:

This  image shows observations taken with the NASA/ ESA Hubble Space Telescope of two magnified galaxies behind massive galaxy clusters: the pink glowing haloes reveal the gas surrounding the distant galaxies and its structure. The gravitational lensing effect from the clusters multiplies the images of the galaxies, producing up to 4 images of the same source.

Trying to parse the caption, I think that the two objects in the center are magnified images of the galaxies in the background, and so should be related to the source of the light in the Einstein ring. But they are yellow and the ring is pink.
Question: So the Einstein ring is picking up light not from the background galaxies, but from a halo of gas around them? Is there a better source to read about this image and everything that's going on here?
The image contains the following label at the top right corner:

click for full size:

The article then says:

The MUSE instrument on the European Southern Observatory's Very Large Telescope has identified halos around distant galaxies. Previously, images of these glowing rings lacked detail, but astronomers were able to use a gravitational lensing effect of galaxy clusters to see detail and structure.
Gravitational lensing occurs as an effect of Albert Einstein's theory of general relativity: mass bends light. So the gravitational fields of galaxy clusters bend light, acting like a magnifier of the galaxies behind them.
The best observations of the halos by MUSE were released Monday, presented by Adélaïde Claeyssens during the annual meeting of the European Astronomical Society in Lyon, France. Claeyssens is a Ph.D. student at the Centre de Recherche Astrophysique de Lyon.

Which further confuses me because the caption mentions Hubble but the article mentions data from MUSE at the VLT. I wonder if the image is a file photo and only peripherally related to the content of the news article?


texts: How many asteroids are named after musicians? Is there a searchable list for this?

texts:
Spot checking Wikipedia, Google and JPL's Horizions I found several musicians right away, including Lou Reed, Elvis, Freddy Mercury, and David Bowie.

Is there a way to find all of the asteroids that are named after musicians somehow? Or at least most of them? If there's not a compiled list, at least something for which an automated search for "music" or "musician" or related terms can be done?


texts: Planets and moons positions in cartesian coordinates?

texts: Is there freely available and reliable source, where one can find positions of planets and moons of the Solar system at given time in cartesian coordinates? (I am writing a simple gravity simulator and it would save me a lot of work with conversion.)


texts:
What places on Pluto and Charon are facing each other? Are Pluto and Charon perfectly tidally locked, or is there a slow rotation over time? And how long after formation should they have become tidally locked? Before or after the late heavy bombardment?

EDIT to add the image series of Pluto below, found at University Today


texts:
Just like binary stars can a system of three stars mutually equidistant from each other?
What I mean is three stars at the vertices of a equilateral
If yes than what will be the orbit of a planet out there
If no what will be the problem for it's nonexistence?


texts: Why does Earth have more gravitational force than the Moon if I stand on it?

texts:
Perhaps this is a silly question but I am trying to understand Einstein's general theory of relativity.

I would like to explain my question in more detail.

The greater the mass of an object, the greater the curvature of spacetime.

If I imagine the curvature of spacetime as a hole with a steep slope around it, I can picture that the closer I get to the center of the hole, the steeper the slope is and therefore the more I would be pushed into the center of the hole. I am not completely sure if that's a very good analogy but in general I think I get the picture of timespace curvature as depicted here:

Now, my question is why is it that the size of the hole matters if I am not standing on the edge (side) of the hole or in the hole but rather on an object that 'covers' the hole (Earth).

Let's say if I stand on a bridge that covers a 20-meter-wide hole and on a bridge the covers 500-meter-wide hole. Wouldn't the gravitational force experienced be the same since the surface on both bridges is flat?


texts: Is the Sun slightly blue in the center? - Wavelength-dependent limb darkening of the Sun

texts:
I've slightly modified the title to try to attract some attention. If we call sunlight "white" and limb darkening is a result of seeing deeper at normal incidence and shallower at oblique incidence, then the center end edge of the solar disk viewed from earth should appear to have slightly different effective color temperatures, since the temperature is varying rapidly with depth.

I'd like to have an approximate expression for the wavelength-dependent limb darkening of the sun in the visible spectrum, either a relatively simple analytical expression that I can understand (with the appropriate coefficients for the sun) or just some linear images of the sun in various bands in the visible wavelengths so I can try to make one of my own.

This question received this helpful comment which links to here but honestly I can't make my way through that to a practical expression I can use. The Wikipedia article is not helping me much either, except for the image there. I plotted scans of RGB but by the time an image gets into the internet, things like sRGB and gamma mean it may not be linear.

update: At the Solar Dynamics Observatory (SDO) website, I just found the image sdo.gsfc.nasa.gov/assets/img/latest/latest_1024_HMIIC.jpg. The color gradient of the limb darkening seems very similar to the Wikimedia image below. I've discovered that it is called a "colorized intensitygram" and the color gradient is purely artificial - the data is single channel intensity. The limb darkening is certainly real (compare to the artificially "flattented" display!)

From http://www.solarham.net/latest_imagery/hmi1.htm

I appreciate (the existence of) the complexities and intricacies of photon transport, instrumental effects, and color perception, but I am just starting to do astronomically correct animations, so please for right now, something imperfect, or not absolutely correct is good enough for me.

20 pixel wide averages of horizontal (-) and vertical (--) "scans" across the center of this 600x600 pixel image from here

note: It seems this is a somewhat futile example, as the image is likely to be a monochrome continuum image with false color!


texts: What is the faintest magnitude a telescope can see?

texts:
This website claims:

  The space telescope can detect objects as faint as 31st magnitude...

It's referring to Hubble, but does not cite any source or math. How do you calculate the faintest (apparent) magnitude that a space telescope can detect?


texts: About coordinate systems and angle differences

texts:
If I want to measure the beam-width of a radio telescope sweeping through a ~ puntual object (the Sun) with a ~ constant flux output when measured:

The value measured sweeping in one frame of reference (and coordinate system), will be the same if it's measured using another frame, right?

If I point to the Sun and sweep in alt-az, I suspect that measurement will be at least roughly similar to a measurement w.r.t e.g., a star on the plane of the Milky Way (using l,b). This because the same principle should be applied for a radio (or any other signal) emitter: a measured angle difference $\Delta \theta$ should be the same in another frame.

What puzzles me is to think that an arbitrary frame could scale any measured quantity but I'm not sure at all.

EDIT
This could be seen in stellarium as:

I apologize if this is a weird, vague or naive question.


texts:
I was hoping that astronomy enthusiasts/scientists would help me out with a simple question about light. Hold your outstretched hand up to any bright light source (the sun, or even a light bulb) so that the light is completely blocked by at least one finger, and then slowly move your finger to the side until the light just peeks around. There seems to be a glow when you do this, and I was wondering what causes this.

Is the light bending around my finger from some sort of diffraction? What's going on?


texts:
I'm writing a piece on Hawking radiation, and find I have something of a problem. The "given" explanation which I find on Wikipedia and elsewhere is unsatisfactory:

  "Physical insight into the process may be gained by imagining that particle–antiparticle radiation is emitted from just beyond the event horizon. This radiation does not come directly from the black hole itself, but rather is a result of virtual particles being "boosted" by the black hole's gravitation into becoming real particles[10]. As the particle–antiparticle pair was produced by the black hole's gravitational energy, the escape of one of the particles lowers the mass of the black hole[11]. An alternative view of the process is that vacuum fluctuations cause a particle–antiparticle pair to appear close to the event horizon of a black hole. One of the pair falls into the black hole while the other escapes. In order to preserve total energy, the particle that fell into the black hole must have had a negative energy..."

It relies on virtual particles and an negative-energy particles. However vacuum fluctuations are not the same thing as virtual particles, which only exist in the mathematics of the model, and we know of know negative-energy particles. So I'm looking for a better explanation. The Wikpedia article also says this:

  "In another model, the process is a quantum tunnelling effect, whereby particle–antiparticle pairs will form from the vacuum, and one will tunnel outside the event horizon[10]."

However that suggests pair production is occurring inside the event horizon, which seems to disregard the infinite gravitational time dilation, and that one of them a) appears outside of the event horizon and b) escapes as Hawking radiation when pair production typically involves the creation of an electron and a positron. Again it's unsatisfactory. So:

Is there a better explanation of Hawking radiation?


texts: Is a branch on HR diagram a line formed by stars in the same evolutionary phase, or the evolutionary track of an individual star?

texts:
Here is a HR diagram from Wikipedia, in the well-known form:

Here is another one from the Stellar evolution article:

I know that when stars leave the main sequence, they go to the giant branch through the subgiant branch. I am a bit confused of what we mean when we say "branch". Looking at the first image, we might think that the subgiant branch is the approximate line of stars formed by different subgiant stars on the HR diagram. While looking at the second one, we might think that the expression branch means the evolutionary track of an individual star.
Which one is it?


texts: How bright is Comet PANSTARRS C/2017 S3 now?

texts:
The LiveScience article Incredible Hulk? Nah, This Glowing, Green Light in the Night Sky Is a Comet published just hours ago discusses Comet PANSTARRS C/2017 S3 and mentions:

  bright bursts exploding from its surface twice in close succession — first on June 30, and then again about two weeks later, Sky and Telescope reported. As Hulk himself might say, "Comet flash!"

The linked Sky and Telescope article PanSTARRS Comet, Rocked by Outburst, Goes Green is dated July 17, 2018 and offers the tantallizing information that

  Comet PanSTARRS (C/2017 S3) has erupted again! Now bright enough to see in binoculars, it might become a naked-eye object if it survives until perihelion.

Question: How bright is the comet now, and how is it predicted to behave in the next week or two (baring further outbursts)?

below: "On July 15, Comet PanSTARRS (C/2017 S3) appeared bright and well condensed with a narrow ion tail." Photo Credit: Michael Jäger. From here.

update: The Universe Today's article Catch Comet C/2017 S3 PANSTARRS In Outburst links to S Seiichi Yoshida's page for the comet, but there's no recent data!


texts: Acquirable Raw Data in Amateur Astrophotography

texts: What raw data can I possibly acquire from an 8" Classical Dobsonian Telescope, and a DSLR? Could anything eye-opening to amateur astronomers be computed or calculated first-hand with such equipment? I'm sure scientists must've considered this equipment "advanced technology" at some point in history not too far back...Could I rediscover or calculate some Laws (like Kepler's laws) or some other things amateur astronomers would be amazed to calculate themselves (like the distance to a planet) using this equipment?


texts:
How did this 'black fabric' that we call space start? How was it made? How was the initial singularity formed? How did the first atom come into existence?

If it all started from a tiny particle that expanded and is now still expanding, how did that tiny particle come into existence ?


texts: Are there other pictures of Titan surface from Huygens?

texts:
I can't find any other picture of the Titan surface from the Huygens probe than this one

Are there any? Are they not disclosed on purpose?

If this is the only one taken, why?

Titan surface http://i.picresize.com/images/2016/11/17/uXaDQ.jpg


texts: What enhances the capture and merge rates of pairs of small black holes orbiting around supermassive black holes?

texts:
I just tried to read the new New York Times article Two Black Holes Colliding Not Enough? Make It Three which links to the new 25-June-2020 Physical Review Letter Graham et al. Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational-Wave Event S190521g
The event described is the merger of two black holes that were embedded in the accretion disk of a supermassive black hole in the center of a galaxy; i.e. in a quasar (loosely speaking).
The NYTimes article describes the following and references a previous paper:

Bolstering that hypothesis was the fact that the flare did not become visible until 34 days after the gravitational waves were detected. It would take about that long for any light from a black hole collision to emerge from such a thick disk of gas, according to a model that Dr. Ford and Barry McKernan, her colleague at the American Museum of Natural History, described in a paper last year.
Dr. Ford described the accretion disk as “ a swarm of stars and dead stars, including black holes,” in a Caltech news release.
She added, “These objects swarm like angry bees around the monstrous queen bee at the center. They can briefly find gravitational partners and pair up but usually lose their partners quickly to the mad dance. But in a supermassive black hole’s disk, the flowing gas converts the mosh pit of the swarm to a classical minuet, organizing the black holes so they can pair up.”

That paper is B. McKernan et al 2019 Ram-pressure stripping of a kicked Hill sphere: Prompt electromagnetic emission from the merger of stellar mass black holes in an AGN accretion disk
Question: Is it possible to explain what it is about "the flowing gas (which) converts the mosh pit of the swarm to a classical minuet, organizing the black holes so they can pair up"? What is it about the accretion disk that enhances the capture and merge rates of pairs of small black holes orbiting within it?


texts: What is so special about the Ultima Thule passed by New Horizon spacecraft?

texts:
I read that the New Horizon spacecraft passed by the Ultima Thule. I also got to know that it is a flat, snowman-shaped rock.

What is so special about it?